In view of the high packing densities attainable with multilevel ceramic circuit structures, they have achieved extensive acceptance in the electronics industry for packaging of semiconductor integrated devices, and other elements, as for example see U.S. Pat. No. 3,379,943 granted Apr. 23, 1968 to J. G. Breedlove, U.S. Pat. No. 3,502,520 granted Mar. 24, 1970 to B. Schwartz and U.S. Pat. No. 4,080,414 granted Mar. 21, 1978 to L. C. Anderson et al.
In general, such conventional ceramic structures are formed from ceramic green sheets which are prepared from ceramic "paints" by mixing a ceramic particulate, a thermoplastic polymer (e.g. polyvinylbutyral) and solvents. This "paint" is then cast or spread into ceramic sheets or slips from which the solvents are subsequently volatilized to provide a coherent and self-supporting flexible green sheet, which may be finally fired to drive off the resin and sinter the ceramic particulates together into a densified ceramic substrate.
In the fabrication of multilevel structures, an electrical conductor forming composition is deposited (by spraying, dipping, screening, etc.) in patterns on required green sheets which form component layers of the desired multilevel structure. The component sheets may have via or feedthrough holes punched in them, as required for level interconnection in the ultimate structure. The required number of component green sheets are stacked or superimposed to register on each other in the required order. The stack of green sheets is then compressed or compacted at necessary temperatures and pressures to effect a bond between adjacent layers not separated by the electrical conductor forming pattern. Thereafter, the green sheet laminate is fired to drive off the binders and to sinter the ceramic and metal particulates together into a ceramic dielectric structure having the desired pattern of electrical conductors extending internally therein.
Alumina (Al.sub.2 O.sub.3), because of its excellent insulating properties, thermal conductivity, stability and strength has received wide acceptance as the material of choice for fabrication of such substrates. However, for various high performance application, the relatively high dielectric constant of alumina (.about.10) entails significant signal propagation delays and noise. A further disadvantage of alumina is its relatively high thermal expansion coefficient (.about.65-70.times.10.sup.-7 /.degree.C.) compared to that of silicon semiconductor chips (.about.25-30.times.10.sup.-7 /.degree.C.) which may, in certain cases, result in some design and reliability concerns, particularly where a silicon chip is joined to the substrate with solder interconnections.
A particular disadvantage is the high sintering and maturing temperature of commercial alumina (.about.1600.degree. C.), which restricts the choice of cosinterable conducting metallurgies to refractory metals such as tungsten, molybdenum, platinum, or any combination of these with each other or with certain other metals, which precludes the use of good electrical conductors such as gold, silver, or copper because the latter will be molten before the sintering temperature of alumina is reached.
A multilayer glass-ceramic structure is disclosed in copending application applications Ser. No. 875,703, filed Feb. 6, 1978 by A. H. Kumar et al; and Ser. No. 23,112 filed Mar. 23, 1979 by L. W. Herron et al now U.S. Pat No. 4,234,367, issued Nov. 18, 1980, (whose teachings are incorporated herein by reference thereto), which eliminates disadvantages of alumina ceramic structures. The disclosed multilayer glass-ceramic structures are characterized with low dielectric constants and are compatible with thick film circuitry of gold, silver or copper, and are co-firable therewith.
Of the two types of glass-ceramics disclosed in the aforesaid application Ser. No. 875,703, one has a .beta.-spodumene, Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2, as the principal crystalline phase while the other has cordierite, 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, as the main crystalline phase. A common feature of these sintered glass-ceramics, among others, is their excellent sinterability and crystallization below 1000.degree. C.
The use of copper is relatively new in the thick film technology. Because of copper's oxidizing potential, it is necessary to sinter multilayer structures in reducing or neutral ambients. However, since reducing ambients can occasion adhesion problems, neutral ambients are preferable. A typical industrial cycle to sinter thick copper films on pre-fired alumina substrate would be at a rate of 50.degree.-70.degree. C./min. to a firing or sintering range of 900.degree.-950.degree. C. with a 15 minute hold at the peak temperature followed by cooling at a rate of 50.degree.-70.degree. C./min.